<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="./jq-3.6.0.js"></script>
</head>

<body>
    <div class="box">
        <div class="item">1</div>
        <div class="item">2</div>
        <div class="item3">3</div>
        <div class="item">4</div>
        <div class="item">5</div>
    </div>
</body>
<script>
    $('.item3').siblings().css({
        'color': 'red',
        'font-size': '30px'
    })
</script>

</html>